Defrosting control in an air-cooling system



J. E. WATKINS Jan. 1, 1952 DEFROSTING CONTROL. IN AN AIR-COOLING SYSTEM 2 SHEETS-SHEET l Filed March 6 lcjoh @mi Jan. 1, 1952 E, WATKlNS 2,580,627

DEFROSTING CONTROL IN AN AIR-COOLING SYSTEM Filed March 6, 1948 2 SHEETS--SHEET 2 N LZ T F22 *Wl/WAH MA "W V A AM 1 "VY" GOS [www ' Ws @rm-@M dohn twat RLYL s Patented Jan. l, 1952 DEFR-OSTING CONTROL IN AN AIR-COCLING SYSTEM John E. Watkins, Maywood, Ill., assignor to Midwest Engineering & Equipment Co., Chicago, Ill., a corporation of Illinois Application March 6, 194.8, Serial No. 13,442

3 Claims.

The invention relates to refrigerating systems generally, and more particularly to refrigerating systems of the flooded evaporator type.

One object of the invention is to provide in a refrigeratng system of the above general character a refrigerant evaporator constructed and related to the other elements of the system in a novel manner which greatly facilitates defrosting.

Another object is to provide an improved waste system for draining olf the water reeased in the defrosting of the evaporator and for preventing the Water from ireezing in other parts of the system during the defrosting operation.

Still another object is to provide a control system whereby periodic defrosting of the refrigerant evaporator is eiiected automatically v. hen required by the condition of/the system.

A further object is to provide a refrigerant evaporator which is simple in construction, which provides a maximum area of heat exchange surface, and which oiers a minimum resistance to the ow of the medium to be cooled, such as air, over the heat exchange surfaces.

Other objects and advantages of the invention will become apparent from the following detailed description of the preferred embodiment illustrated in the accompanying drawings, in which:

Figure 1 is an elevational view partly diagrammatic in character of a refrigerating system embodying the features of the invention.

Fig. 2 is a fragmentary view of a modified form of the system.

Fig. 3 is a sectional view of a control valve of the type employed in the system when automatic controls are provided.

Fig. 4 is a diagrammatic view of the electrical defrosting control system.

While the invention is susceptible oi' various modications and alternative constructions, I have shown in the drawings and will herein describe in detail the preferred embodiment, but it is to be understood that I do not thereby intend to limit the invention to the specific form disclosed, but intend to cover all modications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

For purposes of illustration the invention has been shown in a refrigerating system of the type in which a subfreezing temperature is maintained in a refrigerating room or chamber 5 by continuously circulating the air in the chamber over the heat exchange surfaces provided by a refrigerant evaporator 6. Such circulation of air is effected, in this instance, by a fan or blower 'l driven by an electric motor M. In the exemplary embodiment, the chamber 5 is enclosed by insulated walls 8, the confines of which are ndicated in broken lines in Fig. 1, and the evaporator 6, blower 'l and motor M are enclosed in a compartment 9 closely adjacent the chamber 5 but separated therefrom by insulated walls or partitions l0. As shown in Figs. 1 and 2, the evaporator 6 is mounted with one end projecting through an opening H in the wall or partition I0 to admit air from the chamber 5 into the compartment. After passing through the evaporator and being cooled thereby, the air is discharged by the blower l into the chamber 5 through an outlet opening l2 in the wall l0.

While a refrigerant evaporator of any suitable character may be used in the system, the evaporator 6 herein shown embodies novel features of construction which provide a heat exchange surface of large area, While oiering a minimum of resistance to the flow of air Vthereover. In its preferred form the evaporator 6 comprises an elongated cylindrical sheet metal shell I5 equipped with a plurality of longitudlnally extending tubes I6 xed in suitable tube sheets or heads l1 closing opposite ends of the shell. The tubes I5 may be rolled or welded into the tube sheets and are open at opposite ends to provide passages for the flow of air through the evaporator, the inner surfaces of the tubes defining heat, exchange surfaces for transferring the heat from the circulating air to the refrigerant contained in the evaporator.

When the system is in operation, the evaporator 6 is lled with liquid refrigerant, such as ammonia, to a level 20 somewhat below the top of the evaporator and this level is maintained as the refrigerant evaporates by continuous introduction of fresh liquid refrigerant through an inlet conduit 2| opening into the bottom of the evaporator. The liquid refrigerant may be supplied from any suitable source, such as a conventional compressor-condenser unit having a motor driven compressor 22 adapted to receive spent or gaseous refrigerant from the evaporator and to deliver hot compressed gas through a hot gas line 23 to a condenser 24. The hot gases are cooled and liquefied in the condenser 24 and delivered through a conduit 25, iioat valve FV and conduit 26 to a vertically disposed liquid header 21 which has a, horizontally disposed leg 28 to which the evaporator inlet branch conduit 2| is connected. When required, additional evaporators may be connected to receive refrigerant from asados? the leg 28 through conduits 2|' similar to the conduit 2|.

The spent refrigerant in gaseous form resulting from the evaporation of the refrigerant is drawn off from the evaporator 6 into a suction header 30 which is connected to the suction side of the compressor 22 by a conduit 3|. The suction header 30 is arranged above the evaporator and is connected thereto by a branch conduit 32 opening into the upper end of the evaporator. Preferably, the conduitl 32 is extended upwardly a substantial distance into the suction header to prevent the return iiow of refrigerant which may condense in the header.

As shown in Fig. 1, the liquid header 21 is connected at its upper end to the suction header 30 to provide for the escape of flash gas formed therein. A' conduit 33 connected between the suction header and the valve FV serves to draw off flash gasfrom the valve and to equalize the pressure in the valve housing. It will be understood, of course, that thesuction header 30 may serve a plurality of evaporators similar to the evaporator 6 and similarly connected thereto by conduits 32'. It will also be understood that the compressor system will normally serve a plurality of freezer lines or assemblies which may be similar in all respects to that shown and which are connected to the supply conduit 25 and suction conduit 3| by branch conduits 25 and 3|', respectively.

Provision is made for closing off the inlet and outlet conduits from the evaporator 6 so that the evaporator can be removed for repair or replacement when necessary without interfering with the operation of other evaporators in the system. For this purpose valves 34 and 35 are interposed in the inlet and outlet conduits 2| and 32. These valves may be manually operable valves of any suitable type. Preferably unions 36 and 31 are provided in the respective conduits between the evaporator and the associated valves to facilitate disconnection of the evaporator.

In accordance with the invention, provision is made for defrosting the evaporator 6 in a simple and eiiicient manner and for effectually disposing of the waste water resulting from such defrosting. Defrosting is effected by draining the liquid refrigerant from the evaporator 6 and the liquid header 21 into a refrigerant storage reservoir or sump 40 and introducing hot gas into the evaporator.

In a multiple evaporator system such as that shown, the hot gas may be conveniently obtained from the compressor 22 by way of a hot gas line 4| branching from the conduit 23 between the compressor and the condenser 24 and having a branch or branches 4 I extending to other freezer lines or assemblies when such are installed. It will be appreciated, ofcourse, that the blower 1 is stopped during.T the defrosting operation to interrupt the circulation of air in the chamber and thus avoid unnecessary reduction in the temperature of the chamber.

In order to drain the liquid refrigerant from the system into the sump 4U, the flow of refrigerant through the supply conduit 25 is shut off by closure of a valve 42 interposed in that conduit and the exhaust connection between the suction header 30 and compressor 22 is interrupted by closure of a valve 43 interposed in the suction conduit 3|. The opening of a normally closed valve 44 in a conduit 45 connecting between the leg 28 of the liquid header and the sump 40 permits the refrigerant to drain into the sump. Trapped gas is released from the sump 40 by way of a vent line 46 connecting with the suction header 30 and having interposed therein a valve 41 which is opened at the start of the defrosting operation. Due to the pressure exerted on the refrigerant by the hot gas introduced into the evaporator 6, draining of the system is effected very quickly.

The hot gas in the evaporator quickly melts the frost accumulated in the tubes I6. To handle the water thus produced, the invention provides a waste system which, in conjunction with the novel mounting of the evaporator, effectually prevents spillage of the water on the oor of the chamber 5 and serves to drain the water off to a suitable disposal point outside the chamber. To this end the evaporator 6 is mounted so that the water condensate resulting from the melting of the frost in the tubes I6 is drained to a common point and collected in a waste receptacle 50. To this end theevaporator is supported with its longitudinal axis inclined, as shown in Fig. 1 or, where suflicient head room is available, the evaporator may be vertical, as shown in Fig. 2. In either case, the receptacle 50 is located adjacent the lower end of the evaporator in a position to receive the water draining therefrom.

While the receptacle 50 may be constructed in the form of an elongated trough adapted to cooperate with a plurality of evaporators, it is preferred to construct it as a shallow convex basin 5| adapted to serve a single evaporator.- as shown. Water collected in the basin is discharged through a drain conduit 52 leading to a disposal point outside the chamber 5.

As the receptacle 5l) and at least a part of the drain conduit 52 are exposed at all times to the low temperatures obtaining in the chamber 5, means is provided for applying heatthereto to prevent refreezing of the collected water while the defrosting operation is in progress. While suicient heat for that purpose may be obtained by running the hot gas supply conduit 4| in close proximity to the basin and the drain and enclosing those elements with a common insulating sheath, it is preferred to provide the lower portion of the basin and the ad'jacent portion of the conduit 52 with a gas-tight jacket through which the hot gas used for defrosting may be circulated.

In the exemplary system, the hot gas line 4| has a branch 55 leading to a manifold or distributor 56 which serves to distribute the hot gas to the jackets 53 of the drain receptacles for the various evaporators in the system. During the defrosting operation, hot gas from the header 5B is introduced into the lower end of the jacket 53 by way of a conduit 51 and after passing through the jacket is directed through a conduit 58 to the upper end of the associated evaporator 6. A manually operable valve 59 is provided in the conduit 58 so that it may be shut off when the evaporator is to be disconnected from the system for repair or replacement.

Delivery of hot gas to the jacket 53 and evaporator li is controlled by a valve 60 interposed between the hot gas line 4| and its branch 55. This valve is normally closed and is opened only at the beginning of the defrosting operation. It will be evident that with the above arrangement the waste disposal system is automatically heated whenever not gas is introduced to the evaportor 6 for defrosting the same. Due to the location of the Waste receptacle 5U relative to the evaporator al1 water resulting from the melting of the frost is collected and conducted from the chamber 5, thus avoiding accumulation of ice in the chamber and making unnecessary periodic defrosting of rthe chamber as a whole.

To restore the re.rigerating system to normal operation after defrosting, the valve 60 is closed, thereby shutting oi hot gas from the evaporator and the valve 43 is opened to reconnect the suction header 30 to the compressor. The liquid refrigerant stored in the sump 40 may then be returned to the evaporator. In the exemplary system this is done by closing the valve 41 in vthe sump vent line 46 and opening a valve 6| in a branch of the hot gas line 4| to direct the ow of gas into the sump 40. The pressure of the gas serves to force the liquid refrigerant from the sump through the conduit 45 into the liquid header 21, from which it enters the evaporator by Way of the conduit 2|, as heretofore described.

When the sump 40 is emptied of liquid refrigerant, the valves 44 and 6I are closed and the valve 42 is opened to admit fresh refrigerant into the system under control of the float valve FV. Operation of the system then proceeds in the normal manner.

The various valves of the refrigerating system may be operated manually in the sequence above described to defrost the evaporator and to restore the system to operating condition. However, to obtain maximum operating efficiency and to reduce the cost of supervision, the invention provides a simple and effective control system by which the defrosting and restarting of the refrigerating system is effected automatically when conditions require such action.

In applying such automatic control to the exemplary refrigerating system, the valves 42, 43,

44, 41, B9 and 6I are provided with individually power operated actuators 42S, 43S, 44S, 41S, 60S and BIS, preferably in the form of solenoids. These valves may be of any suitable construction, such as that shown for the Valve 43 in Fig. 3. Thus the Valve has a casing 65 with an internal passage and a movable valve member in the form of a disk E6 for opening or closing the passage. The valve member is normally held in closed position by a spring 61 and is moved to open position by its associated actuator or solenoid 43S, in the case of the valve illustrated. For this purpose the valve member is provided with a stem 6B connected with the solenoid plunger 69 so that the solenoid when energized is operative to lift the valve member from its seat and open the valve passage for the ow of gas or liquid therethrough.

While any suitable means may be utilized to initiate a defrosting operation, it is preferred to employ means which becomes operative in response to a condition of the equipment indicating the need for defrosting, as for example, the accumulation of a predetermined amount of frost on the heat exchange surfaces of the evaporator, that is, within the tubes I6. In the exemplary system, advantage is taken of the fact that the resistance to air ow through the tubes I6 increases progressively as the frost coating is built up, thereby causing a corresponding fall in the current requirements of the blower motor M. Accordingly, means is incorporated in the motor circuit for responding to this change in current flow, such means comprising, in this instance, a current relay RI which becomes energized sutilciently to open normally closed switch contacts Rl-I when the motor current falls to a predetermined value. The opening of the switch contacts` RI--I is utilised to initiate the defrosting operation, as will appear presently.

As shown in Fig. 4 of the drawing, current is supplied to the motor M from a power line comprising the conductors LI, L2 and L3 through switch contacts RZ-I, R2-2 and R2--3 of a motor control relay R2. This relay is energized to start the motor by closure of a manually operable when energized as it is during the normal operation of the refrigerating system closes switch contacts R3-I and R3--2 to complete energizing circuits for the valve solenoids 42S and 43S. The solenoids when energized open their associated valves 42 and 43, as hereinbefore described, to initiate flow of refrigerant to and from the evaporator. Relay R4 when energized opens normally closed switch contacts R4-I and R4-2 for purposes to appear presently.

Deenergization of relay Rl, which occurs upon the accumulation of a predetermined amount of frost on the evaporator tubes as above explained, opens the holding circuits for the relays R2, R3 and R4 and the relays become deenergized. Relay R2 opens switch contacts R2l, R2-2 and R2-3 to stop the motor M and thus interrupt the circulation of air through the evaporator. Relay R3 opens switch contacts R3-I and RGS-2 to interrupt the circuits for the solenoids 42S and 43S, which become deenergized and permit their associated valves to close. The compressor-condenser unit is thus shut oil from the evaporator and the other elements of the refrigerating system associated with the evaporator.

Relay R4 when deenergized closes switch contacts R4-`| to complete circuits for the valve solenoids 44S, 41S and 60S. The circuit for the solenoid 44S includes normally closed switch contacts R5--I of a relay R5 and the circuit for the other solenoids includes normally closed switch contacts RS-I of a relay R6. Relay R4 also closes switch contacts R42 to prepare a reenergizing circuit for the motor control relay R2. Energization of the solenoids 44S, 41S and 60S results in the transfer of the liquid refrigerant from the evaporator 6 to the sump 40, as previously described, and hot gas from the compressor-condenser unit is fed into the accumullator to melt the frost accumulated in the tubes While a timer or the like may be utilized. to terminate the defrosting operation if desired, in order to avoid unnecessarily increasing the tem perature of the refrigerating chamber 5, it is preferred to employ for that purpose a control device 10 responsive to a temperature increase in a selected part of the refrigerating system. As herein shown, the temperature responsive device 10 comprises a bimetal strip 1I supported closely right, as viewed in Fig. 4. and at a predetermined temperature closes switch contacts 12 to complete an energizing circuit for relay R6. Relay R becomes energized, opens switch contacts RS-I to interrupt the circuits vfor solenoids 41S and 50S andclose switch contacts R6-2 to complete a holding circuit for itself, this latter circuit including contacts R4-l of relay R4.

Relay R8 also closes switch contacts RB-L R6-4 and R6-5. Closure of switch contacts R6-4 completes an energizing circuit for valve solenoid 61S by way of switch contacts R5--2 and closure of switch contacts R6-5 completes an energizing circuit for solenoid, 43S. Accordi ingly, at this stage of the cycle. valve 42 is closed shutting olf the supply of liquid refrigerant. Valve 41 closes to seal the vent for the sump 40 and valve 60 closes to interrupt the iiow of hot gas to the evaporator. Valve 43 is opened to reconnect the suction header 30 to the compressor and valve 6I is opened to admit hot gas under pressure to the sump. The liquid refrigerant in the sump is now returned to the evaporator, as explained above.

As the last of the liquid refrigerant is forced out of the sump 4U, a suitable control device, such as a float actuated switch FS, installed in or associated with the sump, closes to complete an energizing circuit for relay R5, which circuit was previously prepared by closure of switch `contacts R6-3. Relay R5 becomes energized and opens switch contacts R5-I and R5-2 to interrept the circuits f-or solenoids 44S and BIS. These solenoids close their respective valves, thus shutting off the sump 40 from the rest of the re-l frigerating system. Connected in parallel with relay R5 and energized simultaneously therewith is a relay R1 which closes switch contacts R1 -l to energize valve solenoid 42S and thus open the refrigerant supply valve 42. Relay R1 also closes switch contacts R1-2 to energize relays R2, R3 and R4, the relays locking up through the switch R2--3 and RI-I and performing their functions exactly as in the initial starting` of the system. Upon energization of relay R4, switch contacts R4-.l and R4-2 are opened to interrupt the circuits for relays R5, R6 and R1 invention provides arefrigerating system of novel and advantageous construction. Defrosting of the system is greatly facilitated by the novel manner in which the evaporator is positioned and related to the other elements of the system, and freezing of the water resulting from defrosting is eflectually prevented by the arrangement for heating the waste system While the defrosting operation is in progress. The evaporator itself is simple in construction and provides a maximum area of heat transfer surface while offering a minimum of resistance to the flow of air thereover.

The invention also provides an improved control system by which defrostinlg of the evaporator may be effected without the intervention of an attendant and at the precise time that such action is required. The system is thus maintained in efllcient operating condition at all times and the cost of supervision is materially reduced. The control system is simple in construction and reliable and foolproof in operation.

I claim as my invention:

l. In a system for maintaining sub-freezing temperatures in a refrigerating chamber. the combination of partitions forming a' compartment adjacent said chamberl for connecting therewith through two openings, a refrigerant evaporator comprising a cylindrical shell dis posed in one of said openings, headers closing the ends of said shell, and tubes extending longitudinally through said shell and through said headers providing passages for the flow of air from said chamber to said compartment,` means for supporting said shell and said tubes in an inclined position, a source of liquid refrigerant. liquid conduit means for supplying liquid refrigerant from said source to said evaporator to cool said tubes below the freezing temperature of wawith hot gas eiiective to melt the frost from said tubes, the liquid condensate resulting from said melted irost flowing along said tubes and said shell to the lower end or said shell, a condensate conduit positioned under said lower` edge to receive said condensate, a gas tight Jacket sur-' rounding a portion of said last mentioned conduit, said hot gas conduit being interconnected with said jacket to cause the hot gas to circulate around said condensate conduit before entering said evaporator, said hot gas serving to heat the' condensate to prevent its freezing in said condensate conduit.

2. a refrigerating system comprising, in combination, a plurality of evaporators including air conduits defining heat exchange surfaces adapted to absorb heat by converting a refrigerant from a liquid to a gaseous state, a compressor, conduit means connected to said compressor and having individual branches connected to each of said evaporators, a condenser connected to the outlet of said compressor for converting to liquid the hot gas discharge under pressure from said compressor, conduit means connected to said condenser and having branches connected weach of said evaporators for conveying liquid to said evaporators, a refrigerant storage sump, conduit means connected to said sump and having` branch lines connected to said evaporators, a hot gas conduit associated with, each evaporator and connecting said respective evaporators with the outlet of said compressor, conduit means interconnecting said sump with the outlet of said compressor, blower means associated with each of said evaporators for effecting forced circulation of air through said air conduits, the moisture in said air collecting on said surfaces in the form of frost, control means associated with each of said evaporators and operative in response to the accumulation of a predetermined quantity of frost in said air conduits and the receptive evaporators for interrupting the operation of the respective blower means and for initiating the defrosting of said receptive evaporators by stopping the flow of fluid from said condenser to said evaporator, opening said evaporator to said sump, and introducing hot gas to said evaporator from said compressor, second control means associated with each evaporator and responsive to an increase in the temperature of the gas in said evaporator undergoing defrosting operation to automatically stop the defrosting action by cutting ofi the supply of hot gas to said evaporator, and introducing hot gas from said compressor into said sump to drive liquid refrigerant back into said evaporator, and a third control associated with said sump and responsive to a reduction in the liquid level in said sump to initiate a refrigerating cycle by closing off the supply of hot gas to said sump, closing off said sump from said evaporator, and by connecting said evaporator to the suction side of said compressor and to the liquid supply of said condenser.

3. In a refrigerating system having a source oi' liquid refrigerant and a source of hot gas under pressure, the combination of a refrigerant evaporator, a temporary liquid storage sump, conduit means connecting said sources with said evaporator and said evaporator with said sump, and valves in said conduit means normally set to admit liquid refrigerant to said evaporator and to prevent the flow of hot gas thereto and to prevent the f ow of liquid from said evaporator to said sump, rst control means operative in response to the accumulation o1' frost on said evaporator for operating a selection of said valves to interrupt the supply of liquid refrigerant to said evaporator, to open said conduit between said evaporator and said sump, and to admit hot gas to said evaporator to drive the liquid from said evaporator into said sump and to defrost said evaporator, conduit means connecting said source of hot gas to said sump, a normally closed 10 valve in said last mentioned conduit means, second control means having a temperature sensitive element in thermal contact with the gas in said evaporator and operative in response to an increase in temperature of the gas in said evaporator beyond a predetermined point for operating a selection of said valves to interrupt the flow of hot gas to said evaporator and to introduce hot gas into said sump to force the liquid therein back into said evaporator, and third control means in contact with the liquid` in said sump and responsive to lowering of the liquid level in said sump for operating said valves to interrupt the supply of hot gas to said sump, to close said conduit between said evaporator and said sump and to re-admit liquid from said source of liquid refrigerant to said evaporator to restore the refrigerating system to its original condition.

JOHN E. WATKINS.

REFERENCES CITED The following references are of record in the le of this patent:

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